1. Field of the Invention
The invention relates to a deflection circuit with a deflection correction circuit. The invention further relates to a display apparatus with such a deflection circuit. Such a deflection circuit is particularly suitable for computer monitors or other display apparatuses which have to display information with substantially different line frequencies.
2. Description of the Related Art
In a display apparatus with a cathode ray tube in which an electron beam is deflected, the angle between the electron beam and the screen of the tube varies during scanning. Therefore, the scanning rate has to be corrected as a function of the position of the point where the beam impinges on the screen. This correction is known as "S-correction", because, when the electron beam is deflected by the electro-magnetical field generated by a deflection coil, the sawtooth-shaped deflection current flowing through the deflection coil has to be corrected to become S-shaped. The S-correction is obtained by an S-capacitor which is arranged in series with the deflection coil.
In a known line deflection circuit, a power supply voltage is supplied to a series arrangement of a choke and a periodically switched electronic switch. A diode, a flyback capacitor, and a series arrangement of the line deflection coil and the S-capacitor are arranged in parallel with the switch. The diode has a polarity so as to be conductive during at least part of a period during which the switch is non-conductive. After the switch has been switched off, the line deflection coil resonates with the flyback capacitor: during the flyback period a high flyback voltage occurs across the flyback capacitor and the current through the line deflection coil reverses sign. At the end of the flyback period, which is the start of the scan period, the diode becomes conductive and the average voltage built up across the S-capacitor occurs across the line deflection coil to cause the S-corrected sawtooth current to run through the line deflection coil. Before the middle of the scan period, where the deflection current changes sign, the switch has to be closed again to maintain the S-capacitor voltage across the line deflection coil. A line period is defined as the sum of the flyback period and the scan period.
In a display apparatus which has to display display signals with substantially different line periods, it is known to adapt the value of the S-capacitor in dependence on the line period. A number of branches is connected in parallel to the S-capacitor, each branch comprising a switch arranged in series with a capacitor. The correct S-capacitance value is approximated by determining, in response to the detected line frequency, which switches should be opened and which switches should be closed. The switches are opened or closed continuously during the period in time that display signals with a certain line frequency are received.
European Patent Application EP-A-823812 discloses that an adjustable S-correction in a line deflection can be obtained by arranging only one switch in series with an extra capacitor arranged in parallel with the S-capacitor. The amount of S-correction is adapted by controlling the on-time of the switch during a line period. In this way it is possible to gradually adjust, in dependence on the line frequency, the S-correction, the line amplitude, and the inner pincushion correction. The inner pincushion distortion is corrected by varying the amount of S-correction as a function of the vertical position.
It is a drawback of the prior art that the shape of the S-correction waveform is a superposition of two or more different parabola functions. It is thus impossible to generate any given shape of the correction waveform within a scan period without using a large amount of parallel branches. A great flexibility in generating the shape of the correction waveform is required to fit a vast variety of cathode ray tubes, each with their specific requirements with respect to the correction waveforms.